Polarity Light is on.....

[sailingdog]

Did you even read what I wrote??? I said that the AC NEUTRAL AND GROUND lines were tied together on the boat. This is a really bad idea and against ABYC specifications IIRC. The AC and DC grounds should be tied together, even if you aren't using a Galvanic Isolator.

If you have no submersed metal, then a galvanic isolator does little for you.

I thought that the common practice IS to connect the ac and dc grounds, PROVIDED you are using a galvanic isolator OR an isolation transformer. Every boat connected to shore power should have some isolation device in my humble opinion.

 

[Old Skool Neil]

Disclaimer: The following is offered without regard to ABYC standards, Calder or perhaps even common boatbuilding practice. But it's still the truth.

In a 120 volt A/C system, the only purpose of the ground is to protect metals that may become inadvertently energized. Providing a sufficient ground path will force the overcurent device to trip thereby rendering the problem to a safe condition.

Example: An onboard microwave has a metal enclosure and is plugged into the boat's shore power system with a 3 prong plug (hot, neutral and ground). Internally, the metal case is connected to the ground. For some reason (chafed wire let's say) the hot wire inside the micro touches the metal casing energizing it. Being grounded and with little or no resistance, the current flow spikes and the circuit breaker trips automatically shutting off the power.

Without the ground wire the breaker doesn't trip and the casing remains energized. The mate, if barefoot and grounded, could grab the micro door handle and complete the circuit with dire consequences.

Side note: Appliances with only a 2 prong plug are not grounded, even if the receptacle they're plugged into is. Without the third prong no ground path is brought to the appliance. This is common with plastic casings.

In practical terms, the only grounds necessary are for the 120 volt appliances connected to the system either by cord and plug or hard wire. DC equipment is not at risk so it need not be grounded to the A/C system.

To echo Dave's sign-off above (Yes I AM a licensed VA electrician) , I'm a licensed CA electrician - nice to meet you, Dave.

 

[Ross]

back in the dark ages electrical plugs were not polarized, both prongs were the same size. But very often the neutral was connected to the chassis. Plugging in incorrectly would energise the chassis and any exposed metal was hot. The polarised plug and the ground prong are there to protect people not equipment. The circuit breaker protects the equipment. Tools that come with plugs that are not polarised are double insulated so that an internal short won't harm the operator unless he has his hand on the drill chuck to guide it more carefully, then all bets are off.

 

[Old Skool Neil]

Ross,
I never intended to suggest the ground protected equipment and re-reading I don't believe I did. The purpose was to prevent any unintended metals from being energized thereby protecting the users.

Sailingdog,
Agreed. The neutral and ground are connected together but only at the main service entrance from the serving utility and nowhere else, not the distribution sub-panels and certainly not the boat.

 

[Blue Nose]

Problem solved! I was at the boat this weekend and thoroughly and methodically went through the system. I checked all recepticles and all were wired correctly. I then realized that the newest peice of equipment added to the boat by the p.o. was a digital multibank battery meter. Logic said start with the most recent since it is doubtful that the boat has been wired incorrectly for 28 years. Sure enough - whoever she had install the meter (probably when the batteries were replaced last year) had cross wired one of the neutrals with one of the hot 22 guage wires coming from the battery meter. This was giving me the Polarity reversed light. Then I noticed a solid copper wire with a capacitor in the middle going from the hot main breaker to another breaker connected to the ground buss. This was causing the main to trip immediately when energized. So the problem was two fold. I correctly connected the battery meter leads but wasn't sure what the capacitor was for. I reconnected that from hot to neutral and everything worked fine.

Does anyone know what the capacitor might be for? It is a smal barrell shaped capacitor (I am presuming) with a stiff copper wire coming out of each end. It was shaped as if it was designed to jump from one breaker to another. It also had the letters OPU on it I believe. Any ideas what it is for and where it really belongs? It probably was moved when the battery meter was installed.

Thanks

 

[Ross]

capacitors are made to block DC and also to provide a phase shift in some ac motors. I can't imagine why they would be in a breaker panel.

 

[Blue Nose]

I am assuming it is a capacitor. Looks like one to me and I had someone elese look at it who knows more than I do and that is what he thought as well.

 

[Snotter]

Is the ground bus strictly for AC use?

 

[Blue Nose]

It appears so. The DC has a separate panel and separate set of busses.

 

[druid]

Concerning connecting neutral and ground: if there is NO connection between the two, you have a very safe system: you need to touch both hot AND neutral to get a shock, and that would PROBABLY be across a finger or two which would PROBABLY not kill you. But if the neutral is connected to ground ANYWHERE (even in the marina), all you have to do is touch hot and be grounded to get a shock(often one finger and your foot, so the current goes through your whole body). That's in addition to the galvanic problems associated with connecting the two.

I agree that you need to know what you're doing before diagnosing and messing with AC systems - that's why electricians need a "ticket". But one point: I beleive the reverse-polarity light will go on if one of the wires is simply not connected (forgotten which one...). If the AC is also "not working" that might be your problem. (Not necessarily completely not connected - maybe just corroded, possibly under insulation or tape)

druid

 

[Old Skool Neil]

To add to Druid's post regarding neutral and ground, neutrals are ALWAYS connected to the ground at the main service entrance (aka main service disconnect) or on the 120/208V (three phase) or 120/240V (single phase) secondary side of a transformer if the main service is of a greater voltage system.